I just purchased this lens along with a 1DsMkIII body, and went out to do some tests. I wanted to compare this to other lenses I have at similar focal lengths. I wanted to determine which is the "go to" lens at certain focal lengths. If I have a shot to take at say 24mm or 35mm - which lens to choose? I tested all at f/11 - the limit where diffraction blurring begins to appear on the 1DsMkIII - and hence a useful setting for large DOF landscape work. I did not compare at wide open apertures since I rarely shoot wide open with wide angle lenses.
At a focal length setting of 24mm the new the 16-35mm outperformed the 24-75mm, 24-105mm, and the 24mm TSE (unfortunately I did not have a 24mm prime to test). The 16-35mm image was the sharpest, had no observable CA (unlike the others), and no observable vignette, and had excellent contrast and saturation. It was a hands down winner. I did not have a 24mm L prime to compare, but it is hard to imagine it being discernibly better. Still, this is a hypothesis to test.
At 35mm was still quite good, but it did show the slightest softness compared to the 24-70mm and the 24-105mm at that focal length. This is logical, since the 16-35mm is at the limit of its design range while the other lenses are into the middle of their range. Still, I was a little surprised to see the difference. It is enough to cause me to switch lenses if I need to shoot close to 35mm for a critical shot.
Across the range of the lens I was impressed by the consistent lack of vignette and distortion as compared to my other L zooms.
I would use this over any of my other lens options in the range from 16mm to around 30mm. Above this the 24-70mm or the 24-105mm begin to dominate in sharpness. Once again, I don't have the 24mm or 35mm primes to compare to.
Although I did not have the 24mm L prime to compare to, I did use the 24mm TSE, which for me is a real mixed bag. The movements are clearly useful for certain situations, but the quality of the optics is just not as good as other Canon lenses IMO (lots of CA), so I think I would tend to use the 16-35mm and go to software tools for solving the problems where the movements are otherwise. If Canon made a version of the 24mm TSE with better optics, and much less vignetting over the width of shift, this would be an option. That lens does not exist however, and probably never will - just not enough market.
In summary, the 16-35mm is an incredible lens, and will be the lens to go to whenever I am in the under 30mm range of focal lengths. At 30-35mm I would be willing to use this, unless the 24-70mm (or perhaps a 35mm prime) were available and the shot was important enough to take the time to swap.
This is probably as close to perfect as a commercially viable wide angle zoom will get however. Highly recommended.